The present invention relates to a reproduction method of an optical disc and a reproduction apparatus.
A DVD (Digital Versatile Disc) is known as an optical disc which can store a large amount of digital information. For DVD, a technique for recording/reproducing data of 4.7 GB on one surface of a disc having a diameter of 12 cm using a red-color laser having a wavelength of about 650 nm has been put into practice. Pits formed on a recording surface of an optical disc each have a width of about 0.3 xcexcm and a length of about 0.4 to 2.0 xcexcm. Such a pit is read by using a laser beam spot having an intensity of full-width at half maximum of about 0.6 xcexcm (see FIG. 3). In a DVD-ROM (Read Only Memory) disc, pits are formed in the shape of oval concavities/convexities. At a stepped portion of a pit, diffraction is caused by an emitted laser beam spot, and a variation in the amount of reflected light is caused due to the diffraction. By utilizing this variation, a reproduction signal is obtained.
On the other hand, the present inventors produced a sample disc on which pits smaller than the pits of the above DVD were formed. The present inventors have been developing a next generation optical disc system where reproduction is performed on such a disc having smaller pits formed thereon, using a beam having a reduced beam spot size, so that higher density and larger capacity are achieved. For example, when a blue-color laser is used as a light source of an optical pick up, the beam spot size is about 0.4 xcexcm, and accordingly, an area density which is two times equal to or higher than that of a conventional DVD can be expected.
Information reproduction on a conventional DVD and the above high-density optical disc under development is described with reference to FIG. 1. FIG. 1 is a schematic diagram showing pits recorded on an optical disc, a laser beam used for reproducing the pits, and a reproduction signal which is reproduced based on the intensity of reflected light. FIG. 1(a) is a drawing of a conventional DVD of low recording density, and FIG. 1(b) is a drawing of the high-density optical disc under consideration.
In the conventional DVD of low recording density, as shown in FIG. 1(a), a reproduction signal 52 is obtained by scanning a pit sequence 50 on the DVD with a red-color laser beam spot 51 for DVD. In this case, when not the entirety but only a portion of the beam spot area overlaps with a pit, the intensity of reflected light is decreased due to diffraction of light. In a reproduction signal 52 which is generated by detecting such a decrease in the intensity of reflected light, the amplitude has a shape that corresponds to the pits 50 and mirror portions 50a (where no pit is formed). In this DVD example, the physical sizes are assumed such that the pit width 53 is about 0.3 xcexcm, and the beam spot size (diameter) 54 is about 0.6 xcexcm.
On the other hand, in the high-density optical disc where pits and a beam are relatively smaller, as shown in FIG. 1(b), a reproduction signal 57 is obtained by scanning a pit sequence 55 with a laser beam spot 56. In this case, when not the entirety but a portion of the beam spot area overlaps with a pit, the intensity of reflected light is decreased due to diffraction of light. In a reproduction signal 57 which is generated by detecting such a decrease in the intensity of reflected light, the amplitude has a shape that corresponds to the pits 55 and mirror portions 55a (where no pit is formed). In this high-density optical disc example, the physical sizes are assumed such that the pit width 58 is about 0.2 xcexcm, and the beam spot size (diameter) 59 is about 0.4 xcexcm. Therefore, in comparison with the conventional DVD, a density higher by a factor of 1.5 is achieved in one-dimensional, i.e., an area density is higher by a factor of 2.25.
The present inventors have been paying attention to a compatibility function with lower-grade mediums, with which a DVD having a conventional density can be reproduced by the above-described next generation high-density optical disc system. We discovered that, when a conventional optical disc is reproduced with an optical pickup with a smaller laser beam spot size, a problem which will be described below is caused because of the relationship in size between a laser beam spot for reproduction and a pit recorded on the disc. This problem is described below with reference to FIG. 2.
FIG. 2 is a schematic diagram showing a number of pits, a laser beam, and a reproduction signal obtained in the case where a conventional low recording density optical disc is reproduced by using a high recording density optical disc system. As shown in FIG. 2, a reproduction signal 62 is obtained by scanning a pit sequence 60, which has been recorded at a conventional recording density, with a laser beam spot 61 for high density information. When the beam spot 61, which has a small diameter, scans a central portion of the pits 60 having a large width w, almost the entire beam spot is included in each pit. In this case, diffraction of light is sufficiently small, and accordingly, the intensity of reflected light is not largely decreased. In the reproduction signal 62, mirror portions 60a (where no pit is formed) are accurately detected, but local peaks 63 in waveforms are detected in the central portions of elongated pits where a weak response (dotted line 65 in FIG. 2) should be obtained because of a reduced intensity of reflected light. When the reproduction signal 62 is converted with a slice level 64 into a binary signal 66, a pseudo pulse 67 is generated in the binary signal 66. The pseudo pulse 67 results in a critical error during reproduction.
In order to prevent generation of such a pseudo pulse, in one method, the beam spot size is increased by decreasing the numerical aperture of an objective lens of an optical pickup or by employing a plurality of lasers which operate at large wavelengths. However, such a method disadvantageously causes complexity of an optical pickup and an increase in cost.
The present invention was conceived in view of the above circumstances. An objective of the present invention is to provide a reproduction method of an optical disc and a reproduction apparatus by which information can be read out without causing an error from an optical disc, on which information has been recorded at a relatively low density, by using a laser having a small beam spot, which is usually used for reproducing a high recording density optical disc.
A reproduction method of an optical disc according to the present invention includes: a step of scanning a pit sequence recorded on the optical disc with a laser beam and detecting an intensity of reflected light from the optical disc by detectors formed by a plurality of light-detecting elements so as to generate detection signals; and a step of generating reproduction data based on the detection signals, wherein
the detectors are grouped into a first light-detecting element group and a second light-detecting element group with respect to a scanning direction of the laser beam, and
the reproduction method includes steps of:
performing an addition operation on a detection signal obtained by the first light-detecting element group and a detection signal obtained by the second light-detecting element group so as to generate a summation signal and outputting the summation signal to a signal processing section;
performing a difference operation on a detection signal obtained by the first light-detecting element group and a detection signal obtained by the second light-detecting element group so as to generate a differential signal and outputting the differential signal to the signal processing section;
digitalizing the summation signal by the signal processing section so as to generate a binary signal, and generating a binary signal edge pulse which corresponds to a rising edge or a falling edge of the binary signal;
only when an amplitude which is equal to or greater than a predetermined value of the differential signal is detected, outputting the binary signal edge pulse as a reproduction edge pulse which corresponds to an edge of the pit from the signal processing section; and
generating the reproduction data based on the reproduction edge pulse, whereby the above objective is achieved.
Where a length of the pit along the scanning direction of the laser beam is longer than a width of the pit along a direction perpendicular to the scanning direction, the width is represented by w, and the intensity of full-width at half maximum of the laser beam spot is represented by h,
when 2wxe2x89xa6h, the binary signal edge pulse is output as it is, from the signal processing section, as the reproduction edge pulse; and
when 2w greater than h, only in the case where an amplitude which is equal to or greater than the predetermined value of the differential signal is detected, the binary signal edge pulse is output from the signal processing section as the reproduction edge pulse.
In one embodiment, the reproduction method of an optical disc further includes a step of determining that the optical disc is a disc for high recording density or a disc for low recording density, wherein:
when it is determined that the optical disc is a disc for high recording density, the binary signal edge pulse is output as it is, from the signal processing section, as the reproduction edge pulse; and
when it is determined that the optical disc is a disc for low recording density, only in the case where an amplitude which is equal to or greater than the predetermined value of the differential signal is detected, the binary signal edge pulse is output from the signal processing section as the reproduction edge pulse.
A reproduction apparatus for an optical disc according to present invention includes an optical pickup including detectors formed by a plurality of light-detecting elements for scanning a pit sequence recorded on the optical disc and detecting an intensity of reflected light from the optical disc so as to generate detection signals, wherein reproduction data is generated based on the detection signals of the detectors,
the detectors are grouped into a first light-detecting element group and a second light-detecting element group with respect to a scanning direction of the laser beam, and
the reproduction apparatus includes:
an adder for performing an addition operation on a detection signal obtained by the first light-detecting element group and a detection signal obtained by the second light-detecting element group so as to generate a summation signal;
a differential amplifier for performing a difference operation on a detection signal obtained by the first light-detecting element group and a detection signal obtained by the second light-detecting element group so as to generate a differential signal; and
a signal processing section for generating a reproduction edge pulse which corresponds to an edge of the pit based on the summation signal and the differential signal, and
the signal processing section includes:
a binarization circuit for digitalizing the summation signal so as t6 generate a binary signal;
an edge detector for generating a binary signal edge pulse which corresponds to a rising edge or a falling edge of the binary signal;
an amplitude detection circuit for outputting a gate signal which is high only when an amplitude which is equal to or greater than a predetermined value of the differential signal is detected;
a logical operation circuit for outputting the binary signal edge pulse as the reproduction edge pulse only when the high gate signal is input thereto; and
a reproduction data generation section for generating the reproduction data based on the reproduction edge pulse, whereby the above objective is achieved.
In one embodiment, the reproduction apparatus for an optical disc further includes a switch for turning ON/OFF an electrical connection between the amplitude detection circuit and the logical operation circuit, wherein the reproduction apparatus is structured such that the switch is turned OFF when the optical disc is a disc for high recording density, and the switch is turned ON when the optical disc is a disc for low recording density.
In one embodiment, the reproduction apparatus for an optical disc further includes a first equalizer for correcting a frequency characteristic of the summation signal and a second equalizer for correcting a frequency characteristic of the differential signal.